1. Field of the Invention
This invention relates generally to disk drives and recordable disks, and more particularly to embedded servo disk drives and their associated disks.
2. Description of the Related Art
Consumer demand has been unrelenting for increased data storage capacity which can be made available to a computer user by a device having a physical profile that is the same or smaller than previous devices which provide less capacity. Even as the ability to store more information in less space is provided, the demand for even greater storage capacity in yet less space arises. There are two ways in which a manufacturer of a data storage unit can increase data storage capacity available to a user of a data storage device without increasing the size the of the storage medium within the device. First, the areal density can be increased. Areal density is the total number of bytes of information that can be stored per square inch of area on a storage medium. Second, the efficiency with which the medium is used can be increased. Efficiency is equal to the number of bytes of information which are available for use by the user, divided by the theoretical number of bytes that could be stored on the media if there were no area lost to overhead.
FIG. 1 is an illustration of the manner in which data is organized on a disk 103 in accordance with one particular type of direct access storage device (DASD), which is commonly utilized in an embedded servo disk drive. In accordance with disk 103 shown in FIG. 1, data is organized in data cells 101 stored within tracks 102 on disk 103. The data is read and written by a read/write head which is suspended over disk 103 as it rotates about a central axis through the center 104 of disk 103 and perpendicular to the plane of disk 103. Each track 102 is comprised of all the information stored on disk 103 at a particular radial distance from center 104 of disk 103. Tracks 102 can be identified either by the radial distance of the track from center 104 of disk 103 or by a track number which is assigned sequentially to each track 102 starting at the track furthest from center 104 of disk 103. In order to use disk 103 in the most efficient manner, the tracks lie in close proximity to one another. For the sake of clarity, however, tracks 102 shown in FIG. 1 are spaced relatively far apart. The width of track 102 is determined by the width of the read/write head and the skew angle of the read/write head (i.e., the angle of the longitudinal axis of the read/write head with respect to a line parallel to the tangent of track 102) and the track misregistration tolerance of the head positioning servo.
Each track 102 is divided by a plurality of conventional servo sample wedges 107 into a plurality of data wedges 105 in which data is stored. Each servo sample wedge 107 consists of a plurality of servo samples which radially extend from the outermost to innermost positions on disk 103 in “servo sectors”. In FIG. 1, disk 103 is shown to have seven (7) servo sample wedges 107. Each one of the servo samples in servo sample wedges 107 includes information used to determine the radial and circumferential position of the read/write head (i.e., the particular track 102 in a particular data wedge 105 over which the read/write head is suspended at each point in time). U.S. Pat. No. 5,285,327 provides information related to servo sectors and is incorporated by herein by reference. The simple servo patterning shown in FIG. 1 (where the servo samples are equally spaced apart and the number of servo samples is constant and independent of the radius) advantageously provides for a simple conventional servo detection scheme which utilizes a single fixed servo sampling frequency for detection.
The portion of a track 102 which lies within one data wedge is hereinafter referred to as a “track wedge” 106. Each data cell 101 on disk 103 typically stores a uniform amount of information (512 bytes, for example). However, the track length (TL) varies as a function of the radial distance of the track from center 104 of disk 103. This is better illustrated in FIGS. 2 and 3. As shown in FIG. 2, the length of track portions within data wedges 105 are smaller at radius R3 than at radius R1. In FIG. 3, track portions 302, 304, and 306 at an outer diameter (OD), a middle diameter (MID), and an inner diameter (ID), respectively, on disk 103 are shown in more detail. The squares in FIG. 3 represent servo samples (such as servo sample 308) and the rectangles in FIG. 3 represent data wedges (such as data wedge 310). Each data wedge in FIG. 3 consist of one or more data sectors in which data is stored. FIGS. 4A-4C further illustrate track portions 302, 304, and 306, respectively, with respect to individual data sectors. As shown in FIG. 4A, track portions 302 at the OD have four (4) data sectors between each servo sample. Track portions 304 at the MID in FIG. 4B, however, have only two and a half (2.5) data sectors between each servo sample. In FIG. 4C, track portions 306 carry only two (2) data sectors between each servo sample.
As apparent, customer data storage is reduced at the ID due to the relatively large amount of storage area consumed by the servo sample data. If the number of servo sample wedges around the disk is chosen to be smaller, the area at the ID is advantageously freed up for customer data, but then track following by the read head at the OD becomes more difficult. Proper track following at the OD is already exacerbated by factors such as disk flutter, air turbulence, and servo control non-linearities such as those caused by actuator magnetic fields. These problems cause increased read data errors at the OD. Increased servo sampling around the disk would reduce the track following issues at the OD, but then the problem of decreased storage capacity at the ID would reemerge. If a smaller number of servo samples were positioned at the ID than at the OD in some arbitrary fashion, the complexity of the servo detector in the disk drive would undesirably be increased and make the detector—if even practically viable—more costly.
Accordingly, what are needed are embedded servo patterning methods and apparatus that provide for a relatively large amount of storage capacity at the ID (and less overhead), proper track following at the OD, and a relatively simple and cost-effective servo detection scheme in a disk drive.